The present invention relates to a rack-and-pinion steering apparatus and, more particularly, to a rack support structure of a rack-and-pinion steering apparatus.
A rack-and-pinion steering apparatus has a steering gear constituted by a rack arranged in the lateral direction between the left and right front wheels (wheels to be steered) of an automobile and a pinion provided at one end portion of a steering wheel shaft and meshed with the rack. This rack-and-pinion steering apparatus converts a rotational displacement according to a steering wheel operation into a linear displacement in the direction of the rack axis, thereby turning the front wheels in a desired direction via a steering link mechanism. The apparatus of this type has a simpler arrangement and a better steering performance than those of apparatuses of other types, and is also advantageous in terms of an installation space.
For this reason, the rack-and-pinion steering apparatus of this type has been adopted not only in manual steering apparatuses but also in power steering apparatuses, such as hydraulic or electric power steering apparatuses. Examples are a manual steering apparatus disclosed in GB2115105A, a hydraulic power steering apparatus disclosed in U.S. Pat. No. 4,793,433, and an electric power steering apparatus disclosed in U.S. Pat. No. 4,898,258.
In the rack-and-pinion steering apparatus of this type, the rack to be meshed with the pinion is supported by a rack support structure constituted by a rack support, a spring means, and a press plug. The rack support is a support member consisting of a metal, which is arranged on the bottom surface side of the rack, i.e., the opposite side of the rack from the pinion, and has a guide groove for slidably supporting the rack in the direction of the rack axis. The spring means biases this rack support in the direction in which the rack meshes with the pinion. The press plug, which is also called an adjusting plug, is so screwed as to close the outer end portion of a hole for assembling these rack support parts into a body and applies a predetermined elastic force to the spring means.
A holding function of the rack support, such as a metal support member, which is biased by the spring means allows the rack teeth of the rack to mesh well with the pinion teeth of the pinion. As a result, a rotational displacement according to a steering wheel operation is extracted as a reciprocal motion in the axial direction to turn the front wheels (wheels to be steered).
This rack support is considered important in ensuring a smooth meshed condition between the rack and the pinion and performing a proper and reliable steering operation.
Conventional rack supports having this function are generally formed by using only metals. In recent years, however, rack supports consisting of only resin materials or both resin materials and metals have been proposed. In particular, a rack support of the latter type can stably and reliably support a rack while maintaining stiffness by its metal member. In addition, its resin member arranged on the sliding surface to be brought into contact with a rack can decrease a frictional resistance with the rack, thereby eliminating a noise problem.
The above conventional rack supports, particularly those including metal members, however, generate a metallic sound, so-called rattle noise, during driving on a bumpy road. More specifically, as described above, the rack support is elastically supported by the spring means interposed between the rack support and the press plug for closing the opening portion of the rack support parts assembly hole of the main body and serving as a fixing member. Therefore, the rack support can absorb a slight load variation, such as a set load. However, upon application of a larger load, such as a kickback, onto the rack, the metal support member cannot be prevented from abutting against the press plug, and this makes it difficult to suppress the rattle noise.
For this reason, in the above rack support, a resin sheet pad is inserted on the sliding surface of a metal support main body for supporting the rack, or a cushioning material sheet, such as a resin sheet-like packing, is interposed between the metal support main body and the press plug. As a result, an impact between the metal members is eliminated, and a backlash in the meshed portion between the pinion and the rack is adjusted by the elastic force provided by the press plug and the spring means.
Even when the resin sheet pad or the cushioning material sheet as described above is used, however, if wear or the like is caused in the meshed portion between the rack and the pinion or the sliding portion, the backlash in the meshed portion between the rack and the pinion is increased to pose the problem of rattle noise.